Total Knee Arthroplasty
The knee is the largest joint in the human body. As shown in FIGS. 1 and 7A, the knee 100 includes the lower end of the femur 102 (thigh bone), the upper end of the tibia 104 (shin bone), and the inner surface of the patella 106 (knee cap, see FIG. 7A). The femur rotates over the tibia and the patella glides in a groove on the end of the femur in front (i.e., anterior). The inner surface of these bony components is covered by cartilage 106 that, along with joint fluid, provides smooth motion of the joint and shock absorption. Various ligaments and muscles help in keeping the knee strong and stable.
Arthritis is a condition where the cartilage 106 starts wearing away and the knee becomes stiff and painful. FIGS. 2 and 3A illustrate a knee exhibiting osteoarthritis, including deterioration of the cartilage 106. As the condition worsens, bone can rub against bone, causing even more pain and loss of function. Knee replacement surgery, also known as knee arthroplasty, is the indicated treatment for many arthritic joint conditions and involves replacing the painful joint with an artificial prosthesis. During knee replacement surgery, the proximal end of the tibia and the distal end of the femur are exposed by an incision made in front (i.e., anterior) of the knee. These bony structures are then cut and shaped to accept prosthetic implants, as shown in FIG. 3B. FIG. 4 illustrates alternative views of knee arthroplasty prosthesis, including a polyethylene implant 402 that is often placed on an inner surface of the patella during the procedure.
FIG. 5 illustrates one embodiment of a total knee prosthesis 500 in isolation. The prosthesis 500 typically includes four components:                1. Femoral component 502, made of a metal material and designed to replace the articular surface and subchondral bone of the distal end of the femur 102;        2. Tibial component 504, made of a metal material and designed to replace the subchondral bone of the proximal tibia 104. This component can also be formed from alternative materials, such as polyethylene;        3. Insert 506, typically formed from polyethylene and designed to provide a bearing surface between the femoral and tibial components 502, 504. It is typically fixed over the tibial component 504 by a locking mechanism; and        4. Patella component 402, made from polyethylene as described above and designed to replace the articular surface of the patella.Total Knee Arthroplasty (TKA), when successful, can result in rapid improvement in pain, joint function, and quality of life for a majority of patients.        
The frequent success of the procedure, in combination with an aging population, has caused demand for TKA procedures to increase rapidly. In 2003 alone, 402,100 TKA operations were performed. With a continued rapid increase in demand, there are projected be close to 3.5 million TKA operations in 2030. The increasing demand for this procedure will put a burden on the health care system in at least two ways: (1) by stressing the number of well trained surgeons and (2) by increasing overall expense.
One strategy to help meet this demand is to reduce the required operation time and improve the outcome of TKA procedures. Spending less time during a primary TKA, and eliminating the need for any subsequent revision procedure, will save time and allow for a higher volume of these procedures to be performed.
Recurrent Surgical Problems
Arthritic knee joints usually present with different degrees of deformity and misalignment because of various degradations and changes in cartilaginous and bony components of the joint. Compared to the normal knee anatomy 602 shown in FIG. 6, there are typically two different types of misalignment: (1) varus deformity 604 (i.e., bow legged) and (2) valgus deformity 606 (i.e., knock knees). One important goal of a TKA procedure is to restore optimal biomechanical alignment in a reconstructed joint. This is because a properly aligned joint will have better function, result in higher patient satisfaction, and increase the longevity of the reconstructed joint.
Proper alignment can be achieved by making different cuts in the distal femur and proximal tibia in relation with various planes and axes of these long bones. As shown in FIGS. 7A-7C and 8 illustrate a typical progression of a TKA operation, in which various cuts are made in the femur and tibia to prepare the bones to receive prostheses. To make the cuts, any of a variety of cutting guides (also known as cutting blocks or jigs) are typically placed next to the bones after they are exposed. The cutting guides provide a reference plane for cutting the bone using a special saw. As can be expected, the cutting blocks have to be precisely positioned at proper angles relative to the bone to ensure correct placement of the prostheses and subsequent alignment of the reconstructed joint. Accordingly, any of a variety of alignment devices can be employed to position the cutting guides, including, for example, the femoral intramedullary alignment device 900 shown in FIG. 9 and the tibial extra-medullary alignment device 1000 shown in FIG. 10.
Unfortunately, most alignment devices available today and designed for this purpose are either not precise enough, very expensive, or both. Indeed, the bone cuts, and consequently the reconstructed joint alignment, is often not as it was intended to be when using these devices.
Currently Available Options
There are three classes of alignment devices currently available:                1. Mechanical instruments, including intramedullary or extra-medullary devices like those shown in FIGS. 9 and 10;        2. Computerized navigation systems, which utilize specialized computers, stereoscopic cameras, and marker structures to track the three-dimensional positioning of objects attached to the marker structures; and        3. Custom-made cutting blocks that align with the shape of a specific patient's bones in a manner that results in correctly oriented bone cuts.        
More recently, another option has been introduced that involves creating a custom set of prostheses for each particular patient's anatomy. All of these options, however, suffer from the problems mentioned above—that is, they are extremely expensive, too imprecise, or both. In addition, the patient-specific devices and prostheses can require additional time and visitation with a surgeon to image the patient's anatomy, design the custom components, and fabricate them in advance of a TKA procedure.